The present invention relates to a method for providing ultraacoustic transducers of the line curtain or point matrix type and the transducers obtained therefrom, having multiple vibrating elements completely separated and acoustically decoupled from each other.
In the present state of art, particularly in the field of ultrasonic visualization, for example, for the medical diagnostics and the echography and holography tests, technical efforts are aimed at showing the image in real time or at least in a very short time.
The scanning techniques proposed in recent years are generally based on the use of multielement line curtain or point matrix transducers. By means of such a type of transducers it is possible to carry out not only an electronic scanning of the acoustic beam, but also a dynamic focusing thereof in order to increase the image resolution.
The scanning techniques carried out in this field are numerous. However, such a treatment, even if it is summary, is not considered as the object of the present invention, in which an ultraacoustic transducer and a method for its realization is exclusively illustrated. As is known an ultraacoustic transducer is a device which converts acoustic energy to electric emergy and viceversa. Such a device is based on physical processes which utilize the interaction between an electric or magnetic field and the matter of the transducer.
The multielement transducers for the ultrasonic visualization utilize the first type of interaction and this is due to the small dimensions of the single elements which have to be comparable to the wave lengths involved, which are of the order of millimeters or fractions thereof.
Although multielement transducers utilizing the electrostatic effect have recently been proposed, the present description will treat only the transducers made by electrostrictive materials (i.e. piezoelectric ceramics) or piezoelectric crystals (for example lithium niobate), since they have been used to a greater extent due to their greater sensitivity.
Both materials show the piezoelectric effect which, as is known, causes a deformation of the material to which an electric field is applied, or vice versa it generates a quantity of charge on the surface of the material which is subjected to a mechanic deformation. This property permits the generation and reception of acoustic waves. The choice of one of both types of materials depends on many factors, and particularly on the technology used for manufacturing the transducer. The techniques by which multielement transducers of the line curtain and/or point matrix type have hitherto been manufactured are essentially of three types.
As far as the first technique is concerned, each vibrating member of the transducer consists of a bar of piezoelectric material having suitable dimensions.
The bars are aligned on the same support, while the emitting surface are covered with a plate of epoxy resin which acts either as impedance adapter or to protect the single vibrators and then to create a monolithic and impermeable transducer. This technique is employed for the line curtain transducers.
Another technique which has thus far been employed is based on cutting more or less deeply, i.e. up to 93% of the thickness, a plate of piezoelectric material so as to obtain linear or punctiform emitting areas. Also in this case the plate rests on a suitable base support and is protected by an epoxy resin.
Finally a technique based on the effect known as trapping of the acoustic energy has recently been proposed. Electrode assemblies having the form of parallel strips are deposited and photoengraved on a plate of piezoelectric material. The acoustic insulation among the various elements is obtained by operating at an intermediate frequency between the resonance frequency of the vibrating mode by thickness dilatation of the area covered with the electrode and the resonance frequency relevant to the uncovered area. This causes a decay of the dominant component of the vibrating mode in the non-metalized area while going away from the metalized one.
This technique, even though it is interesting from a technological point of view, is limited by the fact that the distance between the electrodes is bound to the acoustic decoupling degree to be obtained and then it is not an independent parameter which can freely be chosen by the designer of the visualization system.
From a technological point of view the most difficult problem to be solved for providing a multielement transducer by means of the described techniques is the connection of the electrodes to any type of substrate on which more rigid electric connectors are fixed.
This connection can be carried out by the techniques developed for the thick and thin film technology. However the type of usable piezoelectric material is conditioned by the choice of these techniques. In fact these materials lose, as is known, their characteristic of piezoelectricity at a temperature near the limit temperature of Curie which is typical of each material. It is not necessary to heat the piezoelectric substrate only if the technique of ultrasonic welding of a wire is used. This technique, however, is rather delicate and not highly reliable. Furthermore, as in the case of the thermocompression welding, single connections are necessary between the electrode and the substrate. It is plain that a technique of wire-connection can be used for the construction of line assemblies only and not for point assemblies, in which each single line would be connected by wire to the single connector pins embedded in the substrate.
The characteristics of some modern piezoelectric materials are listed in the following table. All materials are ceramic except the lithium niobate which is a growth crystal.
______________________________________ LiNbO.sub.3 PbNb.sub.2 0.sub.6 PZT5A PZT7A ______________________________________ Relative dielectric constant 30 300 1700 425 Piezoelectric constant (10.sup.-12 m/V) 6 85 374 150 Piezoelectric constant (10.sup.-3 V .multidot. m/N) 22.6 32 24.8 39.9 Coupling factor (%) 16 -- 75.2 66 Q-factor (mechanic) -- 15 75 600 Density (g/cm.sup.3) 4.64 6.2 7.75 7.6 Curie temperature 1210 400 365 350 (.degree.C.) ______________________________________
The PZT5A and the PbNb.sub.2 O.sub.6 have good characteristics both in tramission and in reception, but their Curie points are rather low. The lithium niobate shows a high Curie point but its transmission efficiency is rather low.
In the echographic applications ceramic materials have to be chosen, as high efficiency both in transmission and in reception is needed.
In the holographic systems it is on the other hand possible to use the lithium niobate crystal since the transducer is generally only used in reception. With this crystal, owing to its high Curie point, more advanced and industrialized connecting techniques developed for the production of integrated circuits have been employed. In such a case a structure of the sandwich type is utilized. It consists of a substrate on which an integrated circuit supplying a preprocessed signal can eventually be deposited and which is provided with protuberances of soldering material in a matrix arrangement being juxtaposed to the appropriately engraved plate of piezoelectric material. By heating the substrate and the piezoleectric plate up to about 200.degree. C. under vacuum conditions and by putting on it a moderate pressure a quite good electric connection is obtained.
As already mentioned this technique, which is very attractive as it can be automated, is not suitable on the one hand for the ceramic materials due to their low Curie point and on the other hand it does not allow a visual inspection except by infrared monitors. Finally the single elements of the transducer are not loaded with materials which absorb, in a suitable manner, the acoustic radiation whereby the band width can not be wide. On the contrary this characteristic is important principally in the devices for echographic visualization.